A storage system typically comprises one or more storage devices into which information may be entered, and from which information may be obtained, as desired. The storage system includes a storage operating system that functionally organizes the system by, inter alia, invoking storage operations in support of a storage service implemented by the system. The storage system may be implemented in accordance with a variety of storage architectures including, but not limited to, a network-attached storage environment, a storage area network and a disk assembly directly attached to a client or host computer. The storage devices are typically disk drives organized as a disk array, wherein the term “disk” commonly describes a self-contained rotating magnetic media storage device. The term disk in this context is synonymous with hard disk drive (HDD) or direct access storage device (DASD).
Storage of information on the disk array is preferably implemented as one or more storage “volumes” of physical disks, defining an overall logical arrangement of disk space. The disks within a volume are typically organized as one or more groups, wherein each group may be operated as a Redundant Array of Independent (or Inexpensive) Disks (RAID). RAID implementations enhance the reliability/integrity of data storage through the redundant writing of data “stripes” across a given number of physical disks in the RAID group, and the appropriate storing of redundant information (parity) with respect to the striped data. As described herein, a volume typically comprises at least one data disk and one associated parity disk (or possibly data/parity partitions in a single disk) arranged according to a RAID 4 or equivalent high-reliability implementation. The term “RAID” and its various implementations are well-known and disclosed in A Case for Redundant Arrays of Inexpensive Disks (RAID), by D. A. Patterson, G. A. Gibson and R. H. Katz, Proceedings of the International Conference on Management of Data (SIGMOD), June 1988.
The storage operating system of the storage system may implement a high-level module, such as a file system, to logically organize the information stored on the disks as a hierarchical structure of data containers, such as files and blocks. For example, each “on-disk” file may be implemented as a set of data structures, i.e., disk blocks, configured to store information, such as the actual data for the file. These data blocks are organized within a volume block number (vbn) space that is maintained by the file system. The file system may also assign each data block in the file a corresponding “file offset” or file block number (fbn). The file system typically assigns sequences of fbns on a per-file basis, whereas vbns are assigned over a larger volume address space. The file system organizes the data blocks within the vbn space as a “logical volume”; each logical volume may be, although is not necessarily, associated with its own file system.
A known type of file system is a write-anywhere file system that does not over-write data on disks. If a data block is retrieved (read) from disk into a memory of the storage system and “dirtied” (i.e., updated or modified) with new data, the data block is thereafter stored (written) to a new location on disk to optimize write performance. A write-anywhere file system may initially assume an optimal layout such that the data is substantially contiguously arranged on disks. The optimal disk layout results in efficient access operations, particularly for sequential read operations, directed to the disks. An example of a write-anywhere file system that is configured to operate on a storage system is the Write Anywhere File Layout (WAFL®) file system available from Network Appliance, Inc., Sunnyvale, Calif.
The storage system may be further configured to operate according to a client/server model of information delivery to thereby allow many clients to access data containers stored on the system. In this model, the client may comprise an application, such as a database application, executing on a computer that “connects” to the storage system over a computer network, such as a point-to-point link, shared local area network (LAN), wide area network (WAN), or virtual private network (VPN) implemented over a public network such as the Internet. Each client may request the services of the storage system by issuing file-based and block-based protocol messages (in the form of packets) to the system over the network. In the case of block-based protocol packets, the client requests (and storage system responses) address the information in terms of block addressing on disk using, e.g., a logical unit number (lun). These block-base protocol packets may comprise SCSI encapsulated in TCP/IP (iSCSI).
In such block-based storage system environments, the luns exported by a storage system are only available by accessing that particular system. It should be noted that the term “lun” as used herein may refer to a logical unit number and/or a logical unit. A noted disadvantage of such environments arises when the storage system suffers an error or otherwise becomes inaccessible due to, e.g., a failure in network connectivity. As luns are only available by accessing the storage system, those luns become inaccessible should the storage system become inaccessible. Such inaccessibility is unacceptable for many users of SANs who require high, e.g., “24×7” data availability.
To improve the availability of luns, storage systems may be coupled together in a cluster with the property that when one storage system fails the other begins servicing data access requests directed to the failed storage system's luns. In such an environment, two storage systems are coupled to form a storage system cluster. Each storage system services data access requests directed to its luns and only services data access requests directed to the other storage system's luns after a failover operation has occurred. During the failover operation, the surviving storage system assumes the identity of the failed storage system by, for example, assigning the failed storage system's Internet Protocol (IP) addresses to network adapters available on the surviving storage system. However, a noted disadvantage of such clusters is that they are limited to two storage systems.
Another noted disadvantage of such clusters is that data access requests directed to a lun may only be serviced by one storage system at any given point in time. Thus, if numerous data access requests are directed to a first storage system in a cluster, the second storage system may sit idle while the first storage system is consumed with data access requests. While the disk subsystem of the first storage system may be able to process and/or retrieve information associated with the data access requests, it is possible that the network protocol stacks on the first storage system may become overwhelmed so that initiators may need to throttle their data access requests.